Solid State Physics Laboratory

About: Solid State Physics Laboratory is a facility organization based out in Delhi, India. It is known for research contribution in the topics: Quantum dot & Dielectric. The organization has 1754 authors who have published 2597 publications receiving 50601 citations.

Light harvesting in photonic crystals revisited: why do slow photons at the blue edge enhance absorption?

Abstract: Light harvesting enhancement by slow photons in photonic crystal catalysts or dye-sensitized solar cells is a promising approach for increasing the efficiency of photoreactions. This structural effect is exploited in inverse opal TiO2 photocatalysts by tuning the red edge of the photonic band gap to the TiO2 electronic excitation band edge. In spite of many experimental demonstrations, the slow photon effect is not fully understood yet. In particular, observed enhancement by tuning the blue edge has remained unexplained. Based on rigorous couple wave analysis simulations, we quantify light harvesting enhancement in terms of absorption increase at a specific wavelength (monochromatic UV illumination) or photocurrent increase (solar light illumination), with respect to homogeneous flat slab of equivalent material thickness. We show that the commonly accepted explanation relying on light intensity confinement in high (low) dielectric constant regions at the red (blue) edge is challenged in the case of TiO2 inverse opals because of the sub-wavelength size of the material skeleton. The reason why slow photons at the blue edge are also able to enhance light harvesting is the loose confinement of the field, which leads to significant resonantly enhanced field intensity overlap with the skeleton in both red and blue edge tuning cases, yet with different intensity patterns.

Femtosecond optical generation and detection of coherent acoustic phonons in GaN single crystals

Abstract: We report our experimental and theoretical studies on the time-resolved generation and detection of coherent acoustic phonons (CAPs) in very high quality bulk GaN single crystals, performed using a femtosecond, two-color, all-optical pump-probe technique. A train of ultraviolet laser pulses with energy above the GaN energy gap induced the transient electronic stress at the GaN surface, responsible for the CAP generation. Subsequent CAP oscillations, propagating without any measurable intrinsic attenuation, were observed by scanning the transient differential reflectivity signal $(\ensuremath{\Delta}R∕R)$ of the near-infrared, far-below-band-gap probe beam. The $\ensuremath{\Delta}R∕R$ CAP oscillation amplitude was of the order of ${10}^{\ensuremath{-}5}--{10}^{\ensuremath{-}6}$ and was dependent only on the pump-photon absorption coefficient spectral characteristics. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope corresponding to $8002\ifmmode\pm\else\textpm\fi{}22\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$---the speed of sound in GaN---while the CAP signal phase was constant within the entire range of our experiments. The above experimental results are in excellent agreement with our theoretical modeling and the published literature data.

Coexistence of interfacial stress and charge transfer in graphene oxide-based magnetic nanocomposites

Abstract: In this paper, the existence of both compressive stress and charge transfer process in hydrothermally synthesized cobalt ferrite–graphene oxide (CoFe2O4/GO) nanocomposites has been established. Transmission electron microscopy results reveal the decoration of CoFe2O4 nanoparticles on GO sheets. Magnetic response of nanocomposites was confirmed from superconducting quantum interference device magnetometer measurement. Optical properties of these nanocomposites were investigated by Raman spectroscopy. The interfacial compressive stress involved in this system has been evaluated from observed blue shift of characteristic G peak of graphene oxide. Increase in the full-width half-maximum value as well as upshift in D and G peaks is clear indications of involvement of charge transfer process between GO sheets and dispersed magnetic nanoparticles. The effect of charge transfer process is quantified in terms of shifting of Fermi energy level of these nanocomposites. This is evaluated from variation in contact surface potential difference using scanning Kelvin probe microscopy. XRD spectra of CoFe2O4/GO confirm the polycrystalline nature of CoFe2O4 nanoparticles. Lattice strain estimated from XRD peaks is correlated with the observed Raman shift.